One very common case is losing a race to open a file that was deleted.
No need to spam the logs with mere ENOENT reports.
Other errors are more significant. Log those with errno, and
add event counters to record them.
Signed-off-by: Zygo Blaxell <bees@furryterror.org>
Extents that extend past EOF will have ipos = (file size rounded up
to next block) and e.end() = (file size not rounded), which fails this
constraint check.
The constraint check is wrong. Remove it for now.
Signed-off-by: Zygo Blaxell <bees@furryterror.org>
The previous commit had both max_transid assigments commented out.
It happens to work because we set max_transid in the constructor and
it doesn't change after that, but it's cleaner to assign it explicitly.
Signed-off-by: Zygo Blaxell <bees@furryterror.org>
When an extent ref is modified, all of the refs in the same metadata
page get the same transid in the TREE_SEARCH_V2 header. This causes
two problems:
- Extents with generation < min_transid are included if they
happen to be referenced by pages with generation >= min_transid.
- Extent refs with generation > max_transid are excluded even
if they reference extents with generation <= max_transid.
Both of these are wrong: the first causes some extents to be repeatedly
scanned, the second causes some extents to not be scanned at all.
Change the TREE_SEARCH_V2 parameters so that Crawl sees all extents
newer than min_transid (i.e. set max_transid to max). The TREE_SEARCH_V2
kernel logic already operates this way, i.e. it fetches every page with
transid >= min_transid and discards newer items if they are too new for
max_transid. Filter strictly by the extent reference generation field
(i.e. the copy of the extent generation that is in the extent reference).
Note this still scans extent data multiple times, but it should now
be exactly once per extent reference. A proper fix for this requires
extent-based scanning instead of extent-ref-based scanning.
Formerly commit 5a8c655fc4 "roots: filter
out obsolete extents from extent refs" which landed in the subvol-threads
branch but not master.
Signed-off-by: Zygo Blaxell <bees@furryterror.org>
Scan the roots tree directly for roots other than 5 (the FS root), and
use btrfs_get_root_transid on root_fd for root 5. This avoids filling
up the root FD cache every time we want a new transid_max. Now the only
reason we open a subvol root FD is to open a file within the subvol.
transid_max may be the same as the FS root's transid, in which case
the search loop is not necessary. Place a counter (transid_max_miss)
to see if we ever need to look at root items. If this counter never goes
above zero, or does so very rarely, we can delete the search loop.
Signed-off-by: Zygo Blaxell <bees@furryterror.org>
Task should not block for extended periods of time.
Remove the RateEstimator::wait_for() in crawl_roots. When crawl_roots
runs out of data, let the last crawl_task end without rescheduling.
Schedule crawl_task again on transid polls if it was not already running.
Signed-off-by: Zygo Blaxell <bees@furryterror.org>
BEESLOGNOTE was intended to combine BEESLOG and BEESNOTE, i.e. write a
log message and set the task status message from a single expression.
With the log levels we would now need several more variants
(BEESLOGNOTEDEBUG, BEESLOGNOTEERR...) or a parameter (BEESNOTELOG(DEBUG,
...)).
Or we give up on the idea. This combination was used only 3 times so far.
The log messages and the note message have different editorial styles.
Remove the three instances of BEESLOGNOTE, and make the BEESLOGNOTE
definition equvalent to BEESLOG at LOG_NOTICE level for consistency.
Signed-off-by: Zygo Blaxell <bees@furryterror.org>
The default constructor makes it more convenient to use Task as a
class member.
The ID is useful to disambiguate Task references.
Signed-off-by: Zygo Blaxell <bees@furryterror.org>
update_monotonic does not reset the counter if a new count is smaller than
earlier counts. Useful when consuming an unsorted stream of eveent counts.
Signed-off-by: Zygo Blaxell <bees@furryterror.org>
Reword log message for discovery of new toxic extents vs. lookup of
previously known toxic extents. Also add the block data (especially
filename) to the discovery message.
Signed-off-by: Zygo Blaxell <bees@furryterror.org>
No public version of bees ever created old-style compressed hash table
entries. Remove the code that supports them.
Signed-off-by: Zygo Blaxell <bees@furryterror.org>
Add a third scan mode with alternative trade-offs.
Benefits: Good sequential read performance. Avoids race conditions
described in https://github.com/Zygo/bees/issues/27. Avoids diverting
scan resources into short-lived snapshots before their long-lived
origin subvols are fully scanned.
Drawbacks: Takes the longest time of the three implemented scan-modes
to free space in extents that are shared between snapshots. Uses the
maximum amount of temporary space.
Signed-off-by: Zygo Blaxell <bees@furryterror.org>
Duplicated code between the different scan modes has slowly been
becoming less and less trivial. Move the code to a method and
make both scan-modes call it.
Signed-off-by: Zygo Blaxell <bees@furryterror.org>
Perf was blaming more than 50% of cycles on TREE_SEARCH_V2. strace
showed 4 TREE_SEARCH_V2 calls for every pread in grow_backward().
Fix by increasing the extent fetch batch size so it is more likely
to include the desired items in the first fetch attempt.
This removes TREE_SEARCH_V2 from the top 10 list of cycle consumers.
Signed-off-by: Zygo Blaxell <bees@furryterror.org>
Prealloc extent sizes were taken from the Extent object and did not
take the file size into account. If a file with a non-4K-aligned
size is preallocated, the resulting dedup fails with an exception
because the size of both ranges of the BeesRangePair do not match.
Limit the size of the replacement hole extent to not extend past the
end of the file.
Signed-off-by: Zygo Blaxell <bees@furryterror.org>
Restartng scans for each transid is a bit aggressive. Scan every 10
transids for a polling rate close to the former BEES_COMMIT_INTERVAL.
Signed-off-by: Zygo Blaxell <bees@furryterror.org>
transid_max is now measured at a single point in the crawl_transid thread.
Move the Crawl deferred logic into BeesRoots so it restarts all crawls
when transid_max increases. Gets rid of some messy time arithmetic.
Change name of Crawl thread to "crawl_master" in both thread name and
log messages.
Replace "Next transid" with "Crawl started".
Signed-off-by: Zygo Blaxell <bees@furryterror.org>
The periodic cache age check was not protected by a lock, so multiple
threads may decide to concurrently clear the cache. This led to
duplicate log messages.
Fix by moving the cache expiry trigger out of FdCache and into Roots,
which knows when transids change and can perform cache clears at exactly
the time they are most relevant, i.e. after something that was deleted
becomes permanently so.
This removes the last references to BEES_COMMIT_INTERVAL, so get rid
of its definition too.
Signed-off-by: Zygo Blaxell <bees@furryterror.org>
Now that the polling interval is up to 30 times faster,
next_transid seems too verbose again.
Make it clearer that the interval quoted in the "Deferring..."
message is the computed transaction polling interval.
Combine "Next transid" and "Restarted crawl" into a single message.
Signed-off-by: Zygo Blaxell <bees@furryterror.org>
Make the crawl polling interval more closely track the commit interval
on the btrfs filesystem. In the future this will provide opportunities
to do things like clear FD caches and stop crawls on deleted subvols,
but triggered by transaction commits instead of arbitrary time intervals.
Rename the "crawl" thread so it no longer has the same name as the "crawl"
task, and repurpose it for dedicated transid polling. Cancel the deletion
of crawl_thread and repurpose it to trigger new crawls and wake up the
main crawl Task when it runs out of data.
Signed-off-by: Zygo Blaxell <bees@furryterror.org>
RateEstimator estimates the rate of external events by sampling a
counter.
Conversion functions are provided to predict the time when the
event counter will be incremented to particular values based on past
observations of the event counter.
Synchronization functions are provided to block a thread until a specific
counter value is reached.
Event polling is supported using the history of previous event counts
to determine the predicted time of the next event. A decay function
emphasizes more recent event history.
Polling delays are bounded by minimum and maximum values in the constructor
parameters.
wait_for() and wait_until() block the calling thread until the target
event count is reached (or the counter is reset). These functions are
not bounded by min_delay or max_delay, and require a separate tread
to call update(). wait_for() waits for the counter to be incremented
from its current value by the given count. wait_until() waits for the
counter to reach an absolute value.
update() counts external events and unblocks threads that are blocked
in wait_for() or wait_until(). If the event counter decreases then it
is reset to the new value.
duration() and time_point() convert relative and absolute event counts
into relative and absolute C++11 time quantities based on the last update
time, last observed event count, and the observed event rate.
Convenience functions seconds_for() and seconds_until() calculate
polling delays for for the desired relative and absolute event counts
respectively. These delays are bounded by max and min delay parameters.
rate() and ratio() provide conversion factors based on the current
estimated event rate.
Signed-off-by: Zygo Blaxell <bees@furryterror.org>
Fix discussion of nodatasum files, clarifying what we can and cannot do.
Get rid of some BEESNOTE and BEESTRACE calls which cannot be observed
(well, BEESNOTE can, but you have to be quick!).
Signed-off-by: Zygo Blaxell <bees@furryterror.org>
Having too many "write a message to the log" primitives is confusing,
and having one that intermittently and silently discards output is even
_more_ confusing.
Replace all BEESINFO with appropriate BEESLOG*s. Usually DEBUG.
Except for one or two that occur too often. Just delete those.
Signed-off-by: Zygo Blaxell <bees@furryterror.org>
Add the new WARN_ON bug in v4.14.
Clarify what happens when bees is run on a kernel that is too old.
Signed-off-by: Zygo Blaxell <bees@furryterror.org>
The data field of BeesBlockData is only interesting to those who want
to debug the BeesBlockData implementation or other battle-tested parts
of bees. Users who want to do this can modify and rebuild the source
to enable the output.
To everyone else, the data field is a huge, ongoing infoleak through
the log.
Don't bother with an option, just output the length of the data field
and nothing else.
Fixes: https://github.com/Zygo/bees/issues/53
Signed-off-by: Zygo Blaxell <bees@furryterror.org>
Since we are now unconditionally rendering the print_fn as a static
string, there is no need for it to be a function. We also need it to
be brief and mostly constant.
Use a string instead. Put the string before the function in the Task
constructor arguments so that the title string appears as a heading in
code, since we are making a breaking API change already.
Drop TASK_MACRO as it is broken by this change, but there is no similar
usage of Task anywhere to make it worth fixing.
Signed-off-by: Zygo Blaxell <bees@furryterror.org>
Move pthread_setname_np to the same place we do pthread_getname_np.
Detect errors in pthread_getname_np--but don't throw an exception
because we would call ourself recursively from the exception handler
when it tries to log the exception.
Fix the order of set_name and the first BEESNOTE/BEESLOG call in threads,
closing small time intervals where logs have the wrong thread name,
and that wrong name becomes persistent for the thread.
Make the main thread's name "bees" because Linux kernel stack traces use
the pthread name of the main thread instead of the name of the process.
Anonymous threads get the process name (usually "bees"). We should not
have any such threads, but we do. This appears to occur mostly during
exception stack unwinding. GCC/pthread bug?
Fixes: https://github.com/Zygo/bees/issues/51
Signed-off-by: Zygo Blaxell <bees@furryterror.org>
Threads from the Task module in libcrucible don't set BeesNote::tl_name.
Even if they did, in Task context the thread name is unspecific to the point
of meaninglessness.
Use the Task::print method as the name for such threads, and be sure
that future Task print functions are designed for that usage.
The extra complexity in BeesNote::get_name() seems preferable to
bombarding pthread_setname_np hundreds or thousands of times per second.
FIXME: we are now calling Task::print() on every BeesNote, which
is effectively unconditionally. Maybe we should have Task::print()
and get_name() return a closure, or just evaluate Task::print() once
and cache it in TaskState, or define Task's constructor with a string
argument instead of the current print_fn closure.
Signed-off-by: Zygo Blaxell <bees@furryterror.org>
This enables bees' thread introspection to use task descriptions in
status and log messages.
BeesNote will be calling Task::current_task() from non-Task contexts,
which means we need to allow Task's shared state pointer to be null.
Remove some asserts that will ruin our day in that case.
Signed-off-by: Zygo Blaxell <bees@furryterror.org>
Silence the three(!) log messages per crawl increment an extra one at
the end of the subvol.
The three critical messages per subvol crawl cycle are:
Next transid in BeesCrawlState <SUBVOL>:0 offset 0x0 transid <A>..<B> started <T> (<AGO>s ago)
Subvol has been completely scanned and a new transaction range will
be created. CrawlState is the state of the old subvol.
Restarted crawl BeesCrawlState <SUBVOL>:0 offset 0x0 transid <B>..<C> started <T+AGO> (0s ago)
Subvol has been restarted. CRawlState is the state of the new subvol.
Deferring next transid in BeesCrawlState <SUBVOL>:0 offset 0x0 transid <B>..<C> started <T+AGO> (0s ago)
Subvol has been completely scanned, but it is too soon to start a
new scan.
Fix the "Restart..." message to use the correct verb tense and to use
the correct BeesCrawlState data.
Signed-off-by: Zygo Blaxell <bees@furryterror.org>
When we find a matching block we attempt to extend ("grow") the matched
pair around the first matching block. This function takes the IO hit of
reading the second extent from each duplicate extent pair. It's also
very slow--too many allocations, too small reads, reads in the wrong
order, an order of magnitude too many calls to TREE_SEARCH_V2, and it
is usually in the top 3 most frequent PERFORMANCE warnings.
Start tracking the running time of grows using the pairforward_ms
and pairbackward_ms counters so that we can compare it to various
replacements.
Signed-off-by: Zygo Blaxell <bees@furryterror.org>
We need a better cache expiration algorithm than "make a copy of
the entire thing, sort it while holding a lock, and delete half
the items in a single burst."
Replace the Lamport clock with a double-linked list. Each insert
or lookup operation moves the affected item to the head of the list.
Each erase operation deletes one single item at the tail of the list.
Also sort out some iterator invalidation nonsense by doing erases before
inserts instead of "insert, erase, find the inserted item again because
we invalidated the found iterator during the erase."
The new implementation adds a second word-sized member to each Value
as well as a copy of the Key. Hopefully the enlarged size is not
a deal-breaker.
Signed-off-by: Zygo Blaxell <bees@furryterror.org>
The mlock runs much faster, probably because the hash fetches are
doing most of the work that mlock does.
It makes bees startup latency for testing smaller, even if it takes more
time in absolute terms.
Signed-off-by: Zygo Blaxell <bees@furryterror.org>
Include a brief description of the two algorithms without getting
into too much detail for an ostensibly temporary feature.
Signed-off-by: Zygo Blaxell <bees@furryterror.org>
There are two subvol scan algorithms implemented so far. The two modes
are unimaginatively named 0 and 1.
0: sorts extents by (inode, subvol, offset),
1: scans extents round-robin from all subvols.
Algorithm 0 scans references to the same extent at close to the same
time, which is good for performance; however, whenever a snapshot is
created, the scan of the entire filesystem restarts at the beginning of
the new snapshot.
Algorithm 1 makes continuous forward progress even when new snapshots
are created, but it does not benefit from caching and will force the
kernel to reread data multiple times when there are snapshots.
The algorithm can be selected at run-time using the -m or --scan-mode
option.
We can collect some field data on these before replacing them with
an extent-tree-based scanner. Alternatively, for pre-4.14 kernels,
we can keep these two modes as non-default options.
Currently these algorithms have terrible names. TODO: fix that, but
also TODO: delete all that code and do scans directly from the extent
tree instead.
Augment the scan algorithms relative to their earlier implementation by
batching multiple extents to scan from each subvol before switching to
a different subvol.
Sprinkle some BEESNOTEs on the Task objects so that they don't
disappear from the thread status output.
Adjust some timing constants to deal with the increased latency from
competing threads.
Signed-off-by: Zygo Blaxell <bees@furryterror.org>
Distribute incoming extents across a thread pool for faster execution
on multi-core, multi-disk environments.
Switch extent enumeration model to scan extent refs consecutively(ish).
Signed-off-by: Zygo Blaxell <bees@furryterror.org>
In both instances the code contained within (or the conditional
compilation surrounding it) is no longer controversial.
Signed-off-by: Zygo Blaxell <bees@furryterror.org>
We need a mechanism for distributing work across processor cores and
disks.
Task implements a simple FIFO/LIFO queue model for executing closures.
Some locking primitives are included (mutex and barrier).
Signed-off-by: Zygo Blaxell <bees@furryterror.org>
Bees will someday rely on features available only in kernel v4.14.
Let's start now by removing workarounds for bugs that were fixed in v4.11.
Signed-off-by: Zygo Blaxell <bees@furryterror.org>
Remove some dead code because dedup-related deadlocks have not been
observed since Linux kernel v4.11.
Preserve rationale of remaining #if 0 block (why we do write/rename
instead of write/fsync/rename) so that people don't try to replace the
"missing" fsync() there.
Signed-off-by: Zygo Blaxell <bees@furryterror.org>
With kernel 4.14 there is no sign of the previous LOGICAL_INO performance
problems, so there seems to be no need to throttle threads using this
ioctl.
Increase the FD cache size limits and scan thread count. Let the kernel
figure out scheduling.
Signed-off-by: Zygo Blaxell <bees@furryterror.org>
BEESNOTE puts a message on the status message stack. BEESINFO logs a
message with rate limiting. The message that was flooding the logs
was coming from BEESINFO not BEESNOTE.
Fix earlier commit which removed the wrong message.
Signed-off-by: Zygo Blaxell <bees@furryterror.org>
We were holding weak refs until the next time the resource ID was used.
This is a bad thing if resource IDs are sparse (e.g. pointers or hashes)
because we'll never see an ID twice.
To fix, determine whether we released the last instance of a resource,
and if so, free its weak ref immediately.
Signed-off-by: Zygo Blaxell <bees@furryterror.org>
The bugs in other parts of the code have been identified and fixed,
so the overprotective locks around shared_ptr can be removed.
Keep the other improvements to the Resource class.
Signed-off-by: Zygo Blaxell <bees@furryterror.org>
This avoids PERFORMANCE warnings when large hash tables are used on slow
CPUs or with lots of worker threads. It also simplifies the code (no
locksets, only one object-wide mutex instead of two).
Fixed a few minor bugs along the way (e.g. we were not setting the dirty
flag on the right hash table extent when we detected hash table errors).
Simplified error handling: IO errors on the hash table are ignored,
instead of throwing an exception into the function that tried to use the
hash table.
Signed-off-by: Zygo Blaxell <bees@furryterror.org>
If two utilities are found, we get commands like
/usr/bin/markdown /usr/bin/markdown_py README.md > README.html
and that doesn't work.
Signed-off-by: Zygo Blaxell <bees@furryterror.org>
Read-only snapshots have always just worked. Remove them from the
"untested" list.
nodatasum (and therefore nodatacow) inodes are simply ignored. This seems
like the right thing to do since deduping a nodatacow extent turns it
into a datacow extent, which seems contrary to administrator wishes
implied by the nodatacow bit. We probably need an option to
override that assumption.
Clarify why converted ext[234] filesystems may cause problems and
the nature of those problems.
Assorted minor editorial changes.
Discuss calculation of the balance limit parameter when ensuring
sufficient metadata space.
Update kernel version bug/fix/feature lists, including LOGICAL_INO_V2.
Annotate kernel workaround list with known kernel versions that make
the workarounds necessary.
Remove reference to 'DEFRAG_RANGE' as bees requires much more control
over data placement than this interface can offer. It's easy enough to
create a new ioctl to implement bees requirements once it's known what
those requirements are.
Signed-off-by: Zygo Blaxell <bees@furryterror.org>
When a toxic extent is discovered, insert the offending hash/address/toxic
entry into the hash table.
When a previously discovered toxic extent is encountered, do nothing,
i.e. allow the offending hash/address/toxic entry in the hash table
to expire.
Previously both inserts were removed from the code, but the former one
is required. The latter prevents bees from forgiving toxic extents
(or any hash matching one) should they be relocated, deleted, or simply
become non-toxic.
Signed-off-by: Zygo Blaxell <bees@furryterror.org>
Interesting things happen when blindly swapping the release-build CCFLAGS
with the debug-build commented-out CCFLAGS. None of these things that
happen are good.
Signed-off-by: Zygo Blaxell <bees@furryterror.org>
Use a static function instead of embedding side-effects in the constructor
of an unrelated class.
Signed-off-by: Zygo Blaxell <bees@furryterror.org>
(cherry picked from commit 85106bd9a9)
After a few hundred subvol threads start running, the inode cache starts
to thrash, and the log gets spammed with messages of the form:
"open_root_nocache <subvolid>: <path>"
Ideally there would be some way to schedule work to minimize inode
thrashing. Until that gets done, just silence the messages for now.
Signed-off-by: Zygo Blaxell <bees@furryterror.org>
With many threads it is inconvenient to reassemble the elided parts of
the dedup src/dst and scan filenames output. Simply output them
unconditionally, and balance the line lengths.
Signed-off-by: Zygo Blaxell <bees@furryterror.org>
If we lose a race and open the wrong file, we will not retry with the
next path if the file we opened had incompatible flags. We need to keep
trying paths until we open the correct file or run out of paths.
Fix by moving the inode flag check after the checks for file identity.
Output attributes in hex to be consistent with other attribute error
messages.
There is no need to report root and file paths separately in the error
message for incompatible flags because we have confirmed the identity of
the file before the incompatible flag error is detected. Other messages
in this loop still output root path and file_path separately because
the identity of 'rv' is unknown at the time these messages are emitted.
Signed-off-by: Zygo Blaxell <bees@furryterror.org>
Keep track of the locking thread so we can see why we are deadlocked
in gdb.
Use a handle type for locks based on shared_ptr. Change the handle type
name to flush out any non-auto local variables.
Signed-off-by: Zygo Blaxell <bees@furryterror.org>
(cherry picked from commit aa0b22d445)
This helps identify causes of the "same physical address in dedup"
exception.
Signed-off-by: Zygo Blaxell <bees@furryterror.org>
(cherry picked from commit cc7b4f22b5)
BLOCK_SIZE_MIN_EXTENT_DEFRAG, BLOCK_SIZE_MIN_EXTENT_SPLIT, and others
are no longer used. Remove them.
Signed-off-by: Zygo Blaxell <bees@furryterror.org>
(cherry picked from commit a3d7032eda)
Add time spent in file create and copy operations to the stats.
Signed-off-by: Zygo Blaxell <bees@furryterror.org>
(cherry picked from commit f01c20f972)
A BEESTRACE closure could throw an exception. Trap those so we don't
end up in terminate().
Signed-off-by: Zygo Blaxell <bees@furryterror.org>
(cherry picked from commit 59660cfc00)
Reads can block indefinitely due to bugs, low io priority, or poor
storage performance. Record the block origin data in the thread state
so we can see which reads are problematic.
Signed-off-by: Zygo Blaxell <bees@furryterror.org>
(cherry picked from commit f56f736d28)
Use () instead of [] when the respective end of the byte range touches
the beginning or end of the file. Also omit the '0' at beginning of
file.
Signed-off-by: Zygo Blaxell <bees@furryterror.org>
(cherry picked from commit 3023b7f57a)
Use a different character to make it easier to search for bytenr ranges
in the logs.
Signed-off-by: Zygo Blaxell <bees@furryterror.org>
(cherry picked from commit d43199e3d6)
This will allow the default size limit for cache objects to be changed
with impunity.
Signed-off-by: Zygo Blaxell <bees@furryterror.org>
(cherry picked from commit 9daa51edaa)
perf blames the SEARCH_V2 ioctl wrapper for a lot of time spent in malloc.
Use a thread_local buffer for ioctl results, and reuse it between runs.
Signed-off-by: Zygo Blaxell <bees@furryterror.org>
(cherry picked from commit e509210428)
Holding file FDs open for long periods of time delays inode destruction.
For very large files this can lead to excessive delays while bees dedups
data that will cease to be reachable.
Use the same workaround for file FDs (in the root_ino cache) that
is used for subvols (in the root cache): forcibly close all cached
FDs at regular intervals. The FD cache will reacquire FDs from files
that still have existing paths, and will abandon FDs from files that
no longer have existing paths. The non-existing-path case is not new
(bees has always been able to discover deleted inodes) so it is already
handled by existing code.
Fixes: https://github.com/Zygo/bees/issues/18
Signed-off-by: Zygo Blaxell <bees@furryterror.org>
check_overflow() will invalidate iterators if it decides there are too
many cache entries.
If items are deleted from the cache, search for the inserted item again
to ensure the iterator is valid.
Increase size of timestamp to size_t.
Signed-off-by: Zygo Blaxell <bees@furryterror.org>
Some whitespace fixes. Remove some duplicate code. Don't lock
two BeesStats objects in the - operator method.
Get the locking for T& at(const K&) right to avoid locking a mutex
recursively. Make the non-const version of the function private.
Signed-off-by: Zygo Blaxell <bees@furryterror.org>
Before:
unique_lock<mutex> lock(some_mutex);
// run lock.~unique_lock() because return
// return reference to unprotected heap
return foo[bar];
After:
unique_lock<mutex> lock(some_mutex);
// make copy of object on heap protected by mutex lock
auto tmp_copy = foo[bar];
// run lock.~unique_lock() because return
// pass locally allocated object to copy constructor
return tmp_copy;
Signed-off-by: Zygo Blaxell <bees@furryterror.org>
Before:
unique_lock<mutex> lock(some_mutex);
// run lock.~unique_lock() because return
// return reference to unprotected heap
return foo[bar];
After:
unique_lock<mutex> lock(some_mutex);
// make copy of object on heap protected by mutex lock
auto tmp_copy = foo[bar];
// run lock.~unique_lock() because return
// pass locally allocated object to copy constructor
return tmp_copy;
Signed-off-by: Zygo Blaxell <bees@furryterror.org>
If we release the lock first (and C++ destructor order says we do), then
the return value will be constructed from data living in an unprotected
container object. That data might be destroyed before we get to the
copy constructor for the return value.
Make a temporary copy of the return value that won't be destroyed by any
other thread, then unlock the mutex, then return the copy object.
Signed-off-by: Zygo Blaxell <bees@furryterror.org>
Zygo Blaxell