linux-aio: fix re-entrant completion processing

Commit 0ed93d8 ("linux-aio: process
completions from ioq_submit()") added an optimization that processes
completions each time ioq_submit() returns with requests in flight.
This commit introduces a "Co-routine re-entered recursively" error which
can be triggered with -drive format=qcow2,aio=native.

Fam Zheng <[email protected]>, Kevin Wolf <[email protected]>, and I
debugged the following backtrace:

  (gdb) bt
  #0  0x00007ffff0a046f5 in raise () at /lib64/libc.so.6
  #1  0x00007ffff0a062fa in abort () at /lib64/libc.so.6
  #2  0x0000555555ac0013 in qemu_coroutine_enter (co=0x5555583464d0) at util/qemu-coroutine.c:113
  #3  0x0000555555a4b663 in qemu_laio_process_completions (s=s@entry=0x555557e2f7f0) at block/linux-aio.c:218
  #4  0x0000555555a4b874 in ioq_submit (s=s@entry=0x555557e2f7f0) at block/linux-aio.c:331
  #5  0x0000555555a4ba12 in laio_do_submit (fd=fd@entry=13, laiocb=laiocb@entry=0x555559d38ae0, offset=offset@entry=2932727808, type=type@entry=1) at block/linux-aio.c:383
  #6  0x0000555555a4bbd3 in laio_co_submit (bs=<optimized out>, s=0x555557e2f7f0, fd=13, offset=2932727808, qiov=0x555559d38e20, type=1) at block/linux-aio.c:402
  #7  0x0000555555a4fd23 in bdrv_driver_preadv (bs=bs@entry=0x55555663bcb0, offset=offset@entry=2932727808, bytes=bytes@entry=8192, qiov=qiov@entry=0x555559d38e20, flags=0) at block/io.c:804
  #8  0x0000555555a52b34 in bdrv_aligned_preadv (bs=bs@entry=0x55555663bcb0, req=req@entry=0x555559d38d20, offset=offset@entry=2932727808, bytes=bytes@entry=8192, align=align@entry=512, qiov=qiov@entry=0x555559d38e20, flags=0) at block/io.c:1041
  #9  0x0000555555a52db8 in bdrv_co_preadv (child=<optimized out>, offset=2932727808, bytes=8192, qiov=qiov@entry=0x555559d38e20, flags=flags@entry=0) at block/io.c:1133
  #10 0x0000555555a29629 in qcow2_co_preadv (bs=0x555556635890, offset=6178725888, bytes=8192, qiov=0x555557527840, flags=<optimized out>) at block/qcow2.c:1509
  #11 0x0000555555a4fd23 in bdrv_driver_preadv (bs=bs@entry=0x555556635890, offset=offset@entry=6178725888, bytes=bytes@entry=8192, qiov=qiov@entry=0x555557527840, flags=0) at block/io.c:804
  #12 0x0000555555a52b34 in bdrv_aligned_preadv (bs=bs@entry=0x555556635890, req=req@entry=0x555559d39000, offset=offset@entry=6178725888, bytes=bytes@entry=8192, align=align@entry=1, qiov=qiov@entry=0x555557527840, flags=0) at block/io.c:1041
  #13 0x0000555555a52db8 in bdrv_co_preadv (child=<optimized out>, offset=offset@entry=6178725888, bytes=bytes@entry=8192, qiov=qiov@entry=0x555557527840, flags=flags@entry=0) at block/io.c:1133
  #14 0x0000555555a4515a in blk_co_preadv (blk=0x5555566356d0, offset=6178725888, bytes=8192, qiov=0x555557527840, flags=0) at block/block-backend.c:783
  #15 0x0000555555a45266 in blk_aio_read_entry (opaque=0x5555577025e0) at block/block-backend.c:991
  #16 0x0000555555ac0cfa in coroutine_trampoline (i0=<optimized out>, i1=<optimized out>) at util/coroutine-ucontext.c:78

It turned out that re-entrant ioq_submit() and completion processing
between three requests caused this error.  The following check is not
sufficient to prevent recursively entering coroutines:

  if (laiocb->co != qemu_coroutine_self()) {
      qemu_coroutine_enter(laiocb->co);
  }

As the following coroutine backtrace shows, not just the current
coroutine (self) can be entered.  There might also be other coroutines
that are currently entered and transferred control due to the qcow2 lock
(CoMutex):

  (gdb) qemu coroutine 0x5555583464d0
  #0  0x0000555555ac0c90 in qemu_coroutine_switch (from_=from_@entry=0x5555583464d0, to_=to_@entry=0x5555572f9890, action=action@entry=COROUTINE_ENTER) at util/coroutine-ucontext.c:175
  #1  0x0000555555abfe54 in qemu_coroutine_enter (co=0x5555572f9890) at util/qemu-coroutine.c:117
  #2  0x0000555555ac031c in qemu_co_queue_run_restart (co=co@entry=0x5555583462c0) at util/qemu-coroutine-lock.c:60
  #3  0x0000555555abfe5e in qemu_coroutine_enter (co=0x5555583462c0) at util/qemu-coroutine.c:119
  #4  0x0000555555a4b663 in qemu_laio_process_completions (s=s@entry=0x555557e2f7f0) at block/linux-aio.c:218
  #5  0x0000555555a4b874 in ioq_submit (s=s@entry=0x555557e2f7f0) at block/linux-aio.c:331
  #6  0x0000555555a4ba12 in laio_do_submit (fd=fd@entry=13, laiocb=laiocb@entry=0x55555a338b40, offset=offset@entry=2911477760, type=type@entry=1) at block/linux-aio.c:383
  #7  0x0000555555a4bbd3 in laio_co_submit (bs=<optimized out>, s=0x555557e2f7f0, fd=13, offset=2911477760, qiov=0x55555a338e80, type=1) at block/linux-aio.c:402
  #8  0x0000555555a4fd23 in bdrv_driver_preadv (bs=bs@entry=0x55555663bcb0, offset=offset@entry=2911477760, bytes=bytes@entry=8192, qiov=qiov@entry=0x55555a338e80, flags=0) at block/io.c:804
  #9  0x0000555555a52b34 in bdrv_aligned_preadv (bs=bs@entry=0x55555663bcb0, req=req@entry=0x55555a338d80, offset=offset@entry=2911477760, bytes=bytes@entry=8192, align=align@entry=512, qiov=qiov@entry=0x55555a338e80, flags=0) at block/io.c:1041
  #10 0x0000555555a52db8 in bdrv_co_preadv (child=<optimized out>, offset=2911477760, bytes=8192, qiov=qiov@entry=0x55555a338e80, flags=flags@entry=0) at block/io.c:1133
  #11 0x0000555555a29629 in qcow2_co_preadv (bs=0x555556635890, offset=6157475840, bytes=8192, qiov=0x5555575df720, flags=<optimized out>) at block/qcow2.c:1509
  #12 0x0000555555a4fd23 in bdrv_driver_preadv (bs=bs@entry=0x555556635890, offset=offset@entry=6157475840, bytes=bytes@entry=8192, qiov=qiov@entry=0x5555575df720, flags=0) at block/io.c:804
  #13 0x0000555555a52b34 in bdrv_aligned_preadv (bs=bs@entry=0x555556635890, req=req@entry=0x55555a339060, offset=offset@entry=6157475840, bytes=bytes@entry=8192, align=align@entry=1, qiov=qiov@entry=0x5555575df720, flags=0) at block/io.c:1041
  #14 0x0000555555a52db8 in bdrv_co_preadv (child=<optimized out>, offset=offset@entry=6157475840, bytes=bytes@entry=8192, qiov=qiov@entry=0x5555575df720, flags=flags@entry=0) at block/io.c:1133
  #15 0x0000555555a4515a in blk_co_preadv (blk=0x5555566356d0, offset=6157475840, bytes=8192, qiov=0x5555575df720, flags=0) at block/block-backend.c:783
  #16 0x0000555555a45266 in blk_aio_read_entry (opaque=0x555557231aa0) at block/block-backend.c:991
  #17 0x0000555555ac0cfa in coroutine_trampoline (i0=<optimized out>, i1=<optimized out>) at util/coroutine-ucontext.c:78

Use the new qemu_coroutine_entered() function instead of comparing
against qemu_coroutine_self().  This is correct because:

1. If a coroutine is not entered then it must have yielded to wait for
   I/O completion.  It is therefore safe to enter.

2. If a coroutine is entered then it must be in
   ioq_submit()/qemu_laio_process_completions() because otherwise it
   would be yielded while waiting for I/O completion.  Therefore it will
   check laio->ret and return from ioq_submit() instead of yielding,
   i.e. it's guaranteed not to hang.

Reported-by: Fam Zheng <[email protected]>
Tested-by: Fam Zheng <[email protected]>
Signed-off-by: Stefan Hajnoczi <[email protected]>
Reviewed-by: Fam Zheng <[email protected]>
Message-id: [email protected]
Signed-off-by: Stefan Hajnoczi <[email protected]>

block/linux-aio.c

@@ -94,9 +94,12 @@ static void qemu_laio_process_completion(struct qemu_laiocb *laiocb)
 
     laiocb->ret = ret;
     if (laiocb->co) {
-        /* Jump and continue completion for foreign requests, don't do
-         * anything for current request, it will be completed shortly. */
-        if (laiocb->co != qemu_coroutine_self()) {
+        /* If the coroutine is already entered it must be in ioq_submit() and
+         * will notice laio->ret has been filled in when it eventually runs
+         * later.  Coroutines cannot be entered recursively so avoid doing
+         * that!
+         */
+        if (!qemu_coroutine_entered(laiocb->co)) {
             qemu_coroutine_enter(laiocb->co);
         }
     } else {