Messages in this thread | | | Subject | Re: [PATCH 2/2] lib/percpu_counter: fix dying cpu compare race | From | "yebin (H)" <> | Date | Tue, 4 Apr 2023 14:40:25 +0800 |
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On 2023/4/4 14:01, Dave Chinner wrote: > On Tue, Apr 04, 2023 at 09:42:06AM +0800, Ye Bin wrote: >> From: Ye Bin <yebin10@huawei.com> >> >> In commit 8b57b11cca88 ("pcpcntrs: fix dying cpu summation race") a race >> condition between a cpu dying and percpu_counter_sum() iterating online CPUs >> was identified. >> Acctually, there's the same race condition between a cpu dying and >> __percpu_counter_compare(). Here, use 'num_online_cpus()' for quick judgment. >> But 'num_online_cpus()' will be decreased before call 'percpu_counter_cpu_dead()', >> then maybe return incorrect result. >> To solve above issue, also need to add dying CPUs count when do quick judgment >> in __percpu_counter_compare(). >> >> Signed-off-by: Ye Bin <yebin10@huawei.com> >> --- >> lib/percpu_counter.c | 11 ++++++++++- >> 1 file changed, 10 insertions(+), 1 deletion(-) >> >> diff --git a/lib/percpu_counter.c b/lib/percpu_counter.c >> index 5004463c4f9f..399840cb0012 100644 >> --- a/lib/percpu_counter.c >> +++ b/lib/percpu_counter.c >> @@ -227,6 +227,15 @@ static int percpu_counter_cpu_dead(unsigned int cpu) >> return 0; >> } >> >> +static __always_inline unsigned int num_count_cpus(void) >> +{ >> +#ifdef CONFIG_HOTPLUG_CPU >> + return (num_online_cpus() + num_dying_cpus()); >> +#else >> + return num_online_cpus(); >> +#endif >> +} >> + >> /* >> * Compare counter against given value. >> * Return 1 if greater, 0 if equal and -1 if less >> @@ -237,7 +246,7 @@ int __percpu_counter_compare(struct percpu_counter *fbc, s64 rhs, s32 batch) >> >> count = percpu_counter_read(fbc); >> /* Check to see if rough count will be sufficient for comparison */ >> - if (abs(count - rhs) > (batch * num_online_cpus())) { >> + if (abs(count - rhs) > (batch * num_count_cpus())) { > What problem is this actually fixing? You haven't explained how the > problem you are fixing manifests in the commit message or the cover > letter. Before commit 5825bea05265("xfs: __percpu_counter_compare() inode count debug too expensive"). I got issue as follows when do cpu online/offline test:
smpboot: CPU 1 is now offline XFS: Assertion failed: percpu_counter_compare(&mp->m_ifree, 0) >= 0, file: fs/xfs/xfs_trans.c, line: 622 ------------[ cut here ]------------ kernel BUG at fs/xfs/xfs_message.c:110! invalid opcode: 0000 [#1] SMP KASAN PTI CPU: 3 PID: 25512 Comm: fsstress Not tainted 5.10.0-04288-gcb31bdc8c65d #8 Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.14.0-0-g155821a1990b-prebuilt.qemu.org 04/01/2014 RIP: 0010:assfail+0x77/0x8b fs/xfs/xfs_message.c:110 Code: 7f 10 84 d2 74 0c 48 c7 c7 0c dc e6 ab e8 e8 1e 52 fd 8a 1d 5e 04 5b 01 31 ff 89 de e8 e9 37 14 fd 84 db 74 07 e8 60 36 14 fd <0f> 0b e8 59 36 14 fd 0f 0b 5b 5d 41 5c 41 5d c3 cc cc cc cc e8 47 RSP: 0018:ffff88810a5df5c0 EFLAGS: 00010293 RAX: ffff88810f3a8000 RBX: 0000000000000201 RCX: ffffffffaa8bd7c0 RDX: 0000000000000000 RSI: 0000000000000000 RDI: 0000000000000001 RBP: 0000000000000000 R08: ffff88810f3a8000 R09: ffffed103edf71cd R10: ffff8881f6fb8e67 R11: ffffed103edf71cc R12: ffffffffab0108c0 R13: ffffffffab010220 R14: ffffffffffffffff R15: 0000000000000000 FS: 00007f8536e16b80(0000) GS:ffff8881f6f80000(0000) knlGS:0000000000000000 CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 CR2: 00005617e1115f44 CR3: 000000015873a005 CR4: 0000000000370ee0 DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000 DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400 Call Trace: xfs_trans_unreserve_and_mod_sb+0x833/0xca0 fs/xfs/xfs_trans.c:622 xlog_cil_commit+0x1169/0x29b0 fs/xfs/xfs_log_cil.c:1325 __xfs_trans_commit+0x2c0/0xe20 fs/xfs/xfs_trans.c:889 xfs_create_tmpfile+0x6a6/0x9a0 fs/xfs/xfs_inode.c:1320 xfs_rename_alloc_whiteout fs/xfs/xfs_inode.c:3193 [inline] xfs_rename+0x58a/0x1e00 fs/xfs/xfs_inode.c:3245 xfs_vn_rename+0x28e/0x410 fs/xfs/xfs_iops.c:436 vfs_rename+0x10b5/0x1dd0 fs/namei.c:4329 do_renameat2+0xa19/0xb10 fs/namei.c:4474 __do_sys_renameat2 fs/namei.c:4512 [inline] __se_sys_renameat2 fs/namei.c:4509 [inline] __x64_sys_renameat2+0xe4/0x120 fs/namei.c:4509 do_syscall_64+0x33/0x40 arch/x86/entry/common.c:46 entry_SYSCALL_64_after_hwframe+0x61/0xc6 RIP: 0033:0x7f853623d91d
I can reproduce above issue by injecting kernel latency to invalidate the quick judgment of “__percpu_counter_compare()”. For quick judgment logic, the number of CPUs may have decreased before calling percpu_counter_cpu_dead() when concurrent with CPU offline. That leads to calculation errors. For example: Assumption: (1) batch = 32 (2) The final count is 2 (3) The number of CPUs is 4 If the number of percpu variables on each CPU is as follows when CPU3 is offline cpu0 cpu1 cpu2 cpu3 31 31 31 31 fbc->count = -122 So at this point, add a check to determine if fbc is greater than 0 abs(count - rhs) = -122 batch * num_ online_ cpus() = 32 * 3 = 96 That is: abs (count rhs)>batch * num_online_cpus() conditions met. The actual value is 2, but the fact that count<0 returns -1 is the opposite.
> We generally don't care about the accuracy of the comparison here > because we've used percpu_counter_read() which is completely racy > against on-going updates. e.g. we can get preempted between > percpu_counter_read() and the check and so the value can be > completely wrong by the time we actually check it. Hence checking > online vs online+dying really doesn't fix any of the common race > conditions that occur here. > > Even if we fall through to using percpu_counter_sum() for the > comparison value, that is still not accurate in the face of racing > updates to the counter because percpu_counter_sum only prevents > the percpu counter from being folded back into the global sum > while it is running. The comparison is still not precise or accurate. > > IOWs, the result of this whole function is not guaranteed to be > precise or accurate; percpu counters cannot ever be relied on for > exact threshold detection unless there is some form of external > global counter synchronisation being used for those comparisons > (e.g. a global spinlock held around all the percpu_counter_add() > modifications as well as the __percpu_counter_compare() call). > > That's always been the issue with unsynchronised percpu counters - > cpus dying just don't matter here because there are many other more > common race conditions that prevent accurate, race free comparison > of per-cpu counters. > > Cheers, > > Dave.
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