Date:	Mon, 11 Nov 2013 22:47:57 +0800
From:	fengguang.wu@...el.com
To:	Shaohua Li <shli@...nel.org>
Cc:	NeilBrown <neilb@...e.de>, LKML <linux-kernel@...r.kernel.org>
Subject: [raid5] kernel BUG at drivers/md/raid5.c:701!

28cc2127527dcba2a0817afa8fd5a33c9e023090 is the first bad commit
commit 28cc2127527dcba2a0817afa8fd5a33c9e023090
Author: Shaohua Li <shli@...nel.org>
Date:   Tue Sep 10 15:37:56 2013 +0800

    raid5: relieve lock contention in get_active_stripe()
    
    get_active_stripe() is the last place we have lock contention. It has two
    paths. One is stripe isn't found and new stripe is allocated, the other is
    stripe is found.
    
    The first path basically calls __find_stripe and init_stripe. It accesses
    conf->generation, conf->previous_raid_disks, conf->raid_disks,
    conf->prev_chunk_sectors, conf->chunk_sectors, conf->max_degraded,
    conf->prev_algo, conf->algorithm, the stripe_hashtbl and inactive_list. Except
    stripe_hashtbl and inactive_list, other fields are changed very rarely.
    
    With this patch, we split inactive_list and add new hash locks. Each free
    stripe belongs to a specific inactive list. Which inactive list is determined
    by stripe's lock_hash. Note, even a stripe hasn't a sector assigned, it has a
    lock_hash assigned. Stripe's inactive list is protected by a hash lock, which
    is determined by it's lock_hash too. The lock_hash is derivied from current
    stripe_hashtbl hash, which guarantees any stripe_hashtbl list will be assigned
    to a specific lock_hash, so we can use new hash lock to protect stripe_hashtbl
    list too. The goal of the new hash locks introduced is we can only use the new
    locks in the first path of get_active_stripe(). Since we have several hash
    locks, lock contention is relieved significantly.
    
    The first path of get_active_stripe() accesses other fields, since they are
    changed rarely, changing them now need take conf->device_lock and all hash
    locks. For a slow path, this isn't a problem.
    
    If we need lock device_lock and hash lock, we always lock hash lock first. The
    tricky part is release_stripe and friends. We need take device_lock first.
    Neil's suggestion is we put inactive stripes to a temporary list and readd it
    to inactive_list after device_lock is released. In this way, we add stripes to
    temporary list with device_lock hold and remove stripes from the list with hash
    lock hold. So we don't allow concurrent access to the temporary list, which
    means we need allocate temporary list for all participants of release_stripe.
    
    One downside is free stripes are maintained in their inactive list, they can't
    across between the lists. By default, we have total 256 stripes and 8 lists, so
    each list will have 32 stripes. It's possible one list has free stripe but
    other list hasn't. The chance should be rare because stripes allocation are
    even distributed. And we can always allocate more stripes for cache, several
    mega bytes memory isn't a big deal.
    
    This completely removes the lock contention of the first path of
    get_active_stripe(). It slows down the second code path a little bit though
    because we now need takes two locks, but since the hash lock isn't contended,
    the overhead should be quite small (several atomic instructions). The second
    path of get_active_stripe() (basically sequential write or big request size
    randwrite) still has lock contentions.
    
    Signed-off-by: Shaohua Li <shli@...ionio.com>
    Signed-off-by: NeilBrown <neilb@...e.de>

:040000 040000 84ab47136c389751c7c08ded47b1761b1bee7184 351047cfe3ac66013fc5c77f23d9bb04f869081d M	drivers
bisect run success

# bad: [86737931c2be292ec985df48f2e7fbafb4467f0e] Merge 'md/master' into devel-hourly-2013111107
# good: [5e01dc7b26d9f24f39abace5da98ccbd6a5ceb52] Linux 3.12
git bisect start '86737931c2be292ec985df48f2e7fbafb4467f0e' '5e01dc7b26d9f24f39abace5da98ccbd6a5ceb52' '--'
# good: [21136946c495b0e1e0f7e25a8de6f170efbdeadf] drm/vmwgfx: fix warning if config intel iommu is off.
git bisect good 21136946c495b0e1e0f7e25a8de6f170efbdeadf
# good: [ee360d688c8e37f81c92039f76bebaddbe36befe] Merge branch 'acpi-assorted'
git bisect good ee360d688c8e37f81c92039f76bebaddbe36befe
# good: [cf0613d242805797f252535fcf7bb019512beb46] Merge branch 'gma500-next' of git://github.com/patjak/drm-gma500 into drm-next
git bisect good cf0613d242805797f252535fcf7bb019512beb46
# good: [feba070dbac6f7b477570e590a7dc960b7b0f784] Merge branch 'pm-sleep'
git bisect good feba070dbac6f7b477570e590a7dc960b7b0f784
# good: [6f092343855a71e03b8d209815d8c45bf3a27fcd] net: flow_dissector: fail on evil iph->ihl
git bisect good 6f092343855a71e03b8d209815d8c45bf3a27fcd
# good: [7d963128c95a790b40ae5bac6af23646ceffcb54] Merge 'drm/drm-next' into devel-hourly-2013111107
git bisect good 7d963128c95a790b40ae5bac6af23646ceffcb54
# bad: [917bb50339fbbea9c5f47d257ea42f9652129c3f] raid1: Replace raise_barrier/lower_barrier with freeze_array/unfreeze_array when reconfiguring the array.
git bisect bad 917bb50339fbbea9c5f47d257ea42f9652129c3f
# bad: [28cc2127527dcba2a0817afa8fd5a33c9e023090] raid5: relieve lock contention in get_active_stripe()
git bisect bad 28cc2127527dcba2a0817afa8fd5a33c9e023090
# good: [09b06d70464536cb3cce7cf1c52f23a321604f62] md: fix calculation of stacking limits on level change.
git bisect good 09b06d70464536cb3cce7cf1c52f23a321604f62
# good: [c0f773604d33616b07d61841463a056a780f5ae7] wait: add wait_event_cmd()
git bisect good c0f773604d33616b07d61841463a056a780f5ae7
# first bad commit: [28cc2127527dcba2a0817afa8fd5a33c9e023090] raid5: relieve lock contention in get_active_stripe()

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