Re: [Qemu-devel] [PATCH] block/linux-aio: acquire AioContext before qemu_laio_process_completions

[Kevin Wolf <kwolf@redhat.com>]

Am 05.09.2018 um 13:23 hat Sergio Lopez geschrieben:
> In qemu_laio_process_completions_and_submit, the AioContext is acquired
> before the ioq_submit iteration and after qemu_laio_process_completions,
> but the latter is not thread safe either.
> 
> This change avoids a number of random crashes when the Main Thread and
> an IO Thread collide processing completions for the same AioContext.
> This is an example of such crash:
> 
>  - The IO Thread is trying to acquire the AioContext at aio_co_enter,
>    which evidences that it didn't lock it before:
> 
> Thread 3 (Thread 0x7fdfd8bd8700 (LWP 36743)):
>  #0  0x00007fdfe0dd542d in __lll_lock_wait () at ../nptl/sysdeps/unix/sysv/linux/x86_64/lowlevellock.S:135
>  #1  0x00007fdfe0dd0de6 in _L_lock_870 () at /lib64/libpthread.so.0
>  #2  0x00007fdfe0dd0cdf in __GI___pthread_mutex_lock (mutex=mutex@entry=0x5631fde0e6c0)
>     at ../nptl/pthread_mutex_lock.c:114
>  #3  0x00005631fc0603a7 in qemu_mutex_lock_impl (mutex=0x5631fde0e6c0, file=0x5631fc23520f "util/async.c", line=511) at util/qemu-thread-posix.c:66
>  #4  0x00005631fc05b558 in aio_co_enter (ctx=0x5631fde0e660, co=0x7fdfcc0c2b40) at util/async.c:493
>  #5  0x00005631fc05b5ac in aio_co_wake (co=<optimized out>) at util/async.c:478
>  #6  0x00005631fbfc51ad in qemu_laio_process_completion (laiocb=<optimized out>) at block/linux-aio.c:104
>  #7  0x00005631fbfc523c in qemu_laio_process_completions (s=s@entry=0x7fdfc0297670)
>     at block/linux-aio.c:222
>  #8  0x00005631fbfc5499 in qemu_laio_process_completions_and_submit (s=0x7fdfc0297670)
>     at block/linux-aio.c:237
>  #9  0x00005631fc05d978 in aio_dispatch_handlers (ctx=ctx@entry=0x5631fde0e660) at util/aio-posix.c:406
>  #10 0x00005631fc05e3ea in aio_poll (ctx=0x5631fde0e660, blocking=blocking@entry=true)
>     at util/aio-posix.c:693
>  #11 0x00005631fbd7ad96 in iothread_run (opaque=0x5631fde0e1c0) at iothread.c:64
>  #12 0x00007fdfe0dcee25 in start_thread (arg=0x7fdfd8bd8700) at pthread_create.c:308
>  #13 0x00007fdfe0afc34d in clone () at ../sysdeps/unix/sysv/linux/x86_64/clone.S:113
> 
>  - The Main Thread is also processing completions from the same
>    AioContext, and crashes due to failed assertion at util/iov.c:78:
> 
> Thread 1 (Thread 0x7fdfeb5eac80 (LWP 36740)):
>  #0  0x00007fdfe0a391f7 in __GI_raise (sig=sig@entry=6) at ../nptl/sysdeps/unix/sysv/linux/raise.c:56
>  #1  0x00007fdfe0a3a8e8 in __GI_abort () at abort.c:90
>  #2  0x00007fdfe0a32266 in __assert_fail_base (fmt=0x7fdfe0b84e68 "%s%s%s:%u: %s%sAssertion `%s' failed.\n%n", assertion=assertion@entry=0x5631fc238ccb "offset == 0", file=file@entry=0x5631fc23698e "util/iov.c", line=line@entry=78, function=function@entry=0x5631fc236adc <__PRETTY_FUNCTION__.15220> "iov_memset")
>     at assert.c:92
>  #3  0x00007fdfe0a32312 in __GI___assert_fail (assertion=assertion@entry=0x5631fc238ccb "offset == 0", file=file@entry=0x5631fc23698e "util/iov.c", line=line@entry=78, function=function@entry=0x5631fc236adc <__PRETTY_FUNCTION__.15220> "iov_memset") at assert.c:101
>  #4  0x00005631fc065287 in iov_memset (iov=<optimized out>, iov_cnt=<optimized out>, offset=<optimized out>, offset@entry=65536, fillc=fillc@entry=0, bytes=15515191315812405248) at util/iov.c:78
>  #5  0x00005631fc065a63 in qemu_iovec_memset (qiov=<optimized out>, offset=offset@entry=65536, fillc=fillc@entry=0, bytes=<optimized out>) at util/iov.c:410
>  #6  0x00005631fbfc5178 in qemu_laio_process_completion (laiocb=0x7fdd920df630) at block/linux-aio.c:88
>  #7  0x00005631fbfc523c in qemu_laio_process_completions (s=s@entry=0x7fdfc0297670)
>     at block/linux-aio.c:222
>  #8  0x00005631fbfc5499 in qemu_laio_process_completions_and_submit (s=0x7fdfc0297670)
>     at block/linux-aio.c:237
>  #9  0x00005631fbfc54ed in qemu_laio_poll_cb (opaque=<optimized out>) at block/linux-aio.c:272
>  #10 0x00005631fc05d85e in run_poll_handlers_once (ctx=ctx@entry=0x5631fde0e660) at util/aio-posix.c:497
>  #11 0x00005631fc05e2ca in aio_poll (blocking=false, ctx=0x5631fde0e660) at util/aio-posix.c:574
>  #12 0x00005631fc05e2ca in aio_poll (ctx=0x5631fde0e660, blocking=blocking@entry=false)
>     at util/aio-posix.c:604
>  #13 0x00005631fbfcb8a3 in bdrv_do_drained_begin (ignore_parent=<optimized out>, recursive=<optimized out>, bs=<optimized out>) at block/io.c:273
>  #14 0x00005631fbfcb8a3 in bdrv_do_drained_begin (bs=0x5631fe8b6200, recursive=<optimized out>, parent=0x0, ignore_bds_parents=<optimized out>, poll=<optimized out>) at block/io.c:390
>  #15 0x00005631fbfbcd2e in blk_drain (blk=0x5631fe83ac80) at block/block-backend.c:1590
>  #16 0x00005631fbfbe138 in blk_remove_bs (blk=blk@entry=0x5631fe83ac80) at block/block-backend.c:774
>  #17 0x00005631fbfbe3d6 in blk_unref (blk=0x5631fe83ac80) at block/block-backend.c:401
>  #18 0x00005631fbfbe3d6 in blk_unref (blk=0x5631fe83ac80) at block/block-backend.c:449
>  #19 0x00005631fbfc9a69 in commit_complete (job=0x5631fe8b94b0, opaque=0x7fdfcc1bb080)
>     at block/commit.c:92
>  #20 0x00005631fbf7d662 in job_defer_to_main_loop_bh (opaque=0x7fdfcc1b4560) at job.c:973
>  #21 0x00005631fc05ad41 in aio_bh_poll (bh=0x7fdfcc01ad90) at util/async.c:90
>  #22 0x00005631fc05ad41 in aio_bh_poll (ctx=ctx@entry=0x5631fddffdb0) at util/async.c:118
>  #23 0x00005631fc05e210 in aio_dispatch (ctx=0x5631fddffdb0) at util/aio-posix.c:436
>  #24 0x00005631fc05ac1e in aio_ctx_dispatch (source=<optimized out>, callback=<optimized out>, user_data=<optimized out>) at util/async.c:261
>  #25 0x00007fdfeaae44c9 in g_main_context_dispatch (context=0x5631fde00140) at gmain.c:3201
>  #26 0x00007fdfeaae44c9 in g_main_context_dispatch (context=context@entry=0x5631fde00140) at gmain.c:3854
>  #27 0x00005631fc05d503 in main_loop_wait () at util/main-loop.c:215
>  #28 0x00005631fc05d503 in main_loop_wait (timeout=<optimized out>) at util/main-loop.c:238
>  #29 0x00005631fc05d503 in main_loop_wait (nonblocking=nonblocking@entry=0) at util/main-loop.c:497
>  #30 0x00005631fbd81412 in main_loop () at vl.c:1866
>  #31 0x00005631fbc18ff3 in main (argc=<optimized out>, argv=<optimized out>, envp=<optimized out>)
>     at vl.c:4647
> 
>  - A closer examination shows that s->io_q.in_flight appears to have
>    gone backwards:
> 
> (gdb) frame 7
>  #7  0x00005631fbfc523c in qemu_laio_process_completions (s=s@entry=0x7fdfc0297670)
>     at block/linux-aio.c:222
> 222	            qemu_laio_process_completion(laiocb);
> (gdb) p s
> $2 = (LinuxAioState *) 0x7fdfc0297670
> (gdb) p *s
> $3 = {aio_context = 0x5631fde0e660, ctx = 0x7fdfeb43b000, e = {rfd = 33, wfd = 33}, io_q = {plugged = 0,
>     in_queue = 0, in_flight = 4294967280, blocked = false, pending = {sqh_first = 0x0,
>       sqh_last = 0x7fdfc0297698}}, completion_bh = 0x7fdfc0280ef0, event_idx = 21, event_max = 241}
> (gdb) p/x s->io_q.in_flight
> $4 = 0xfffffff0
> 
> Signed-off-by: Sergio Lopez <slp@redhat.com>

Thanks, applied to the block branch.

Kevin