block: fix race between request completion and timeout handling

crocode i2c_i801 i2c_core iTCO_wdt iTCO_vendor_support shpchp ioatdma dca be2net sg ses enclosure ext4 mbcache jbd2 sd_mod crc_t10dif ahci megaraid_sas(U) dm_mirror dm_region_hash dm_log dm_mod [last unloaded: scsi_wait_scan]

Pid: 491, comm: scsi_eh_0 Tainted: G        W  ----------------   2.6.32-220.13.1.el6.x86_64 wandboard-org#1 IBM  -[8722PAX]-/00D1461
RIP: 0010:[<ffffffff8124e424>]  [<ffffffff8124e424>] blk_requeue_request+0x94/0xa0
RSP: 0018:ffff881057eefd60  EFLAGS: 00010012
RAX: ffff881d99e3e8a8 RBX: ffff881d99e3e780 RCX: ffff881d99e3e8a8
RDX: ffff881d99e3e8a8 RSI: ffff881d99e3e780 RDI: ffff881d99e3e780
RBP: ffff881057eefd80 R08: ffff881057eefe90 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000000 R12: ffff881057f92338
R13: 0000000000000000 R14: ffff881057f92338 R15: ffff883058188000
FS:  0000000000000000(0000) GS:ffff880040200000(0000) knlGS:0000000000000000
CS:  0010 DS: 0018 ES: 0018 CR0: 000000008005003b
CR2: 00000000006d3ec0 CR3: 000000302cd7d000 CR4: 00000000000406b0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000ffff0ff0 DR7: 0000000000000400
Process scsi_eh_0 (pid: 491, threadinfo ffff881057eee000, task ffff881057e29540)
Stack:
 0000000000001057 0000000000000286 ffff8810275efdc0 ffff881057f16000
<0> ffff881057eefdd0 ffffffff81362323 ffff881057eefe20 ffffffff8135f393
<0> ffff881057e29af8 ffff8810275efdc0 ffff881057eefe78 ffff881057eefe90
Call Trace:
 [<ffffffff81362323>] __scsi_queue_insert+0xa3/0x150
 [<ffffffff8135f393>] ? scsi_eh_ready_devs+0x5e3/0x850
 [<ffffffff81362a23>] scsi_queue_insert+0x13/0x20
 [<ffffffff8135e4d4>] scsi_eh_flush_done_q+0x104/0x160
 [<ffffffff8135fb6b>] scsi_error_handler+0x35b/0x660
 [<ffffffff8135f810>] ? scsi_error_handler+0x0/0x660
 [<ffffffff810908c6>] kthread+0x96/0xa0
 [<ffffffff8100c14a>] child_rip+0xa/0x20
 [<ffffffff81090830>] ? kthread+0x0/0xa0
 [<ffffffff8100c140>] ? child_rip+0x0/0x20
Code: 00 00 eb d1 4c 8b 2d 3c 8f 97 00 4d 85 ed 74 bf 49 8b 45 00 49 83 c5 08 48 89 de 4c 89 e7 ff d0 49 8b 45 00 48 85 c0 75 eb eb a4 <0f> 0b eb fe 0f 1f 84 00 00 00 00 00 55 48 89 e5 0f 1f 44 00 00
RIP  [<ffffffff8124e424>] blk_requeue_request+0x94/0xa0
 RSP <ffff881057eefd60>

The RIP is this line:
        BUG_ON(blk_queued_rq(rq));

After digging through the code, I think there may be a race between the
request completion and the timer handler running.

A timer is started for each request put on the device's queue (see
blk_start_request->blk_add_timer).  If the request does not complete
before the timer expires, the timer handler (blk_rq_timed_out_timer)
will mark the request complete atomically:

static inline int blk_mark_rq_complete(struct request *rq)
{
        return test_and_set_bit(REQ_ATOM_COMPLETE, &rq->atomic_flags);
}

and then call blk_rq_timed_out.  The latter function will call
scsi_times_out, which will return one of BLK_EH_HANDLED,
BLK_EH_RESET_TIMER or BLK_EH_NOT_HANDLED.  If BLK_EH_RESET_TIMER is
returned, blk_clear_rq_complete is called, and blk_add_timer is again
called to simply wait longer for the request to complete.

Now, if the request happens to complete while this is going on, what
happens?  Given that we know the completion handler will bail if it
finds the REQ_ATOM_COMPLETE bit set, we need to focus on the completion
handler running after that bit is cleared.  So, from the above
paragraph, after the call to blk_clear_rq_complete.  If the completion
sets REQ_ATOM_COMPLETE before the BUG_ON in blk_add_timer, we go boom
there (I haven't seen this in the cores).  Next, if we get the
completion before the call to list_add_tail, then the timer will
eventually fire for an old req, which may either be freed or reallocated
(there is evidence that this might be the case).  Finally, if the
completion comes in *after* the addition to the timeout list, I think
it's harmless.  The request will be removed from the timeout list,
req_atom_complete will be set, and all will be well.

This will only actually explain the coredumps *IF* the request
structure was freed, reallocated *and* queued before the error handler
thread had a chance to process it.  That is possible, but it may make
sense to keep digging for another race.  I think that if this is what
was happening, we would see other instances of this problem showing up
as null pointer or garbage pointer dereferences, for example when the
request structure was not re-used.  It looks like we actually do run
into that situation in other reports.

This patch moves the BUG_ON(test_bit(REQ_ATOM_COMPLETE,
&req->atomic_flags)); from blk_add_timer to the only caller that could
trip over it (blk_start_request).  It then inverts the calls to
blk_clear_rq_complete and blk_add_timer in blk_rq_timed_out to address
the race.  I've boot tested this patch, but nothing more.

Signed-off-by: Jeff Moyer <[email protected]>
Acked-by: Hannes Reinecke <[email protected]>
Cc: [email protected]
Signed-off-by: Jens Axboe <[email protected]>

block/blk-core.c

@@ -2227,6 +2227,7 @@ void blk_start_request(struct request *req)
 	if (unlikely(blk_bidi_rq(req)))
 		req->next_rq->resid_len = blk_rq_bytes(req->next_rq);
 
+	BUG_ON(test_bit(REQ_ATOM_COMPLETE, &req->atomic_flags));
 	blk_add_timer(req);
 }
 EXPORT_SYMBOL(blk_start_request);

block/blk-timeout.c

@@ -91,8 +91,8 @@ static void blk_rq_timed_out(struct request *req)
 		__blk_complete_request(req);
 		break;
 	case BLK_EH_RESET_TIMER:
-		blk_clear_rq_complete(req);
 		blk_add_timer(req);
+		blk_clear_rq_complete(req);
 		break;
 	case BLK_EH_NOT_HANDLED:
 		/*
@@ -174,7 +174,6 @@ void blk_add_timer(struct request *req)
 		return;
 
 	BUG_ON(!list_empty(&req->timeout_list));
-	BUG_ON(test_bit(REQ_ATOM_COMPLETE, &req->atomic_flags));
 
 	/*
 	 * Some LLDs, like scsi, peek at the timeout to prevent a