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[PATCH 4.4 41/49] fs/dcache.c: avoid soft-lockup in dput()

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4.4-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Wei Fang <fangwei1@huawei.com>

commit 47be61845c775643f1aa4d2a54343549f943c94c upstream.

We triggered soft-lockup under stress test which
open/access/write/close one file concurrently on more than
five different CPUs:

WARN: soft lockup - CPU#0 stuck for 11s! [who:30631]
...
[<ffffffc0003986f8>] dput+0x100/0x298
[<ffffffc00038c2dc>] terminate_walk+0x4c/0x60
[<ffffffc00038f56c>] path_lookupat+0x5cc/0x7a8
[<ffffffc00038f780>] filename_lookup+0x38/0xf0
[<ffffffc000391180>] user_path_at_empty+0x78/0xd0
[<ffffffc0003911f4>] user_path_at+0x1c/0x28
[<ffffffc00037d4fc>] SyS_faccessat+0xb4/0x230

->d_lock trylock may failed many times because of concurrently
operations, and dput() may execute a long time.

Fix this by replacing cpu_relax() with cond_resched().
dput() used to be sleepable, so make it sleepable again
should be safe.

Signed-off-by: Wei Fang <fangwei1@huawei.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
 fs/dcache.c |    7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

--- a/fs/dcache.c
+++ b/fs/dcache.c
@@ -578,7 +578,6 @@ static struct dentry *dentry_kill(struct
 
 failed:
 	spin_unlock(&dentry->d_lock);
-	cpu_relax();
 	return dentry; /* try again with same dentry */
 }
 
@@ -752,6 +751,8 @@ void dput(struct dentry *dentry)
 		return;
 
 repeat:
+	might_sleep();
+
 	rcu_read_lock();
 	if (likely(fast_dput(dentry))) {
 		rcu_read_unlock();
@@ -783,8 +784,10 @@ repeat:
 
 kill_it:
 	dentry = dentry_kill(dentry);
-	if (dentry)
+	if (dentry) {
+		cond_resched();
 		goto repeat;
+	}
 }
 EXPORT_SYMBOL(dput);
 

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[PATCH 4.4 00/49] 4.4.18-stable review Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2016-08-14 22:30 +0200
  [PATCH 4.4 33/49] mm: memcontrol: fix swap counter leak on swapout from offline cgroup Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2016-08-14 22:30 +0200
  [PATCH 4.4 05/49] net: bgmac: Fix infinite loop in bgmac_dma_tx_add() Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2016-08-14 22:30 +0200
  [PATCH 4.4 26/49] x86/mtrr: Fix Xorg crashes in Qemu sessions Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2016-08-14 22:30 +0200
  [PATCH 4.4 28/49] x86/xen, pat: Remove PAT table init code from Xen Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2016-08-14 22:30 +0200
  [PATCH 4.4 01/49] tcp: make challenge acks less predictable Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2016-08-14 22:30 +0200
  [PATCH 4.4 03/49] ipv4: reject RTNH_F_DEAD and RTNH_F_LINKDOWN from user space Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2016-08-14 22:30 +0200
  [PATCH 4.4 12/49] PNP: Add Broadwell to Intel MCH size workaround Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2016-08-14 22:30 +0200
  [PATCH 4.4 23/49] x86/mm/pat: Add support of non-default PAT MSR setting Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2016-08-14 22:30 +0200
  [PATCH 4.4 24/49] x86/mm/pat: Add pat_disable() interface Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2016-08-14 22:30 +0200
  [PATCH 4.4 10/49] scsi: ignore errors from scsi_dh_add_device() Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2016-08-14 22:30 +0200
  [PATCH 4.4 20/49] apparmor: fix ref count leak when profile sha1 hash is read Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2016-08-14 22:30 +0200
  [PATCH 4.4 41/49] fs/dcache.c: avoid soft-lockup in dput() Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2016-08-14 22:30 +0200
  [PATCH 4.4 49/49] ext4: fix reference counting bug on block allocation error Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2016-08-14 22:30 +0200
  [PATCH 4.4 07/49] qed: Fix setting/clearing bit in completion bitmap Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2016-08-14 22:30 +0200
  [PATCH 4.4 38/49] fuse: fsync() did not return IO errors Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2016-08-14 22:30 +0200
  [PATCH 4.4 46/49] ext4: dont call ext4_should_journal_data() on the journal inode Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2016-08-14 22:30 +0200
  [PATCH 4.4 43/49] crypto: scatterwalk - Fix test in scatterwalk_done Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2016-08-14 22:30 +0200
  [PATCH 4.4 19/49] Revert "s390/kdump: Clear subchannel ID to signal non-CCW/SCSI IPL" Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2016-08-14 22:30 +0200
  [PATCH 4.4 45/49] ext4: fix deadlock during page writeback Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2016-08-14 22:30 +0200
  [PATCH 4.4 22/49] devpts: clean up interface to pty drivers Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2016-08-14 22:30 +0200
  [PATCH 4.4 48/49] ext4: short-cut orphan cleanup on error Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2016-08-14 22:30 +0200
  [PATCH 4.4 32/49] mm: memcontrol: fix cgroup creation failure after many small jobs Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2016-08-14 22:30 +0200
  [PATCH 4.4 06/49] net/irda: fix NULL pointer dereference on memory allocation failure Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2016-08-14 22:30 +0200
  [PATCH 4.4 27/49] x86/mtrr: Fix PAT init handling when MTRR is disabled Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2016-08-14 22:30 +0200
  [PATCH 4.4 44/49] ext4: check for extents that wrap around Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2016-08-14 22:30 +0200
  [PATCH 4.4 29/49] x86/pat: Document the PAT initialization sequence Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2016-08-14 22:30 +0200
  [PATCH 4.4 36/49] block: fix use-after-free in seq file Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2016-08-14 22:30 +0200
  [PATCH 4.4 37/49] sysv, ipc: fix security-layer leaking Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2016-08-14 22:40 +0200
  [PATCH 4.4 09/49] ipath: Restrict use of the write() interface Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2016-08-14 22:40 +0200
  [PATCH 4.4 35/49] x86/syscalls/64: Add compat_sys_keyctl for 32-bit userspace Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2016-08-14 22:40 +0200
  [PATCH 4.4 21/49] random: strengthen input validation for RNDADDTOENTCNT Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2016-08-14 22:40 +0200
  [PATCH 4.4 04/49] bonding: set carrier off for devices created through netlink Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2016-08-14 22:40 +0200
  [PATCH 4.4 11/49] PNP: Add Haswell-ULT to Intel MCH size workaround Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2016-08-14 22:40 +0200
  Re: [PATCH 4.4 00/49] 4.4.18-stable review Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2016-08-15 10:00 +0200
  Re: [PATCH 4.4 00/49] 4.4.18-stable review Guenter Roeck <linux@roeck-us.net> - 2016-08-15 15:10 +0200
  Re: [PATCH 4.4 00/49] 4.4.18-stable review Shuah Khan <shuah.kh@samsung.com> - 2016-08-16 06:10 +0200

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