linux-ext4 - [PATCHv2 1/1] ext4: mballoc: Use this_cpu_read instead of this_cpu

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Date:   Tue,  9 Jun 2020 16:23:10 +0530
From:   Ritesh Harjani <riteshh@...ux.ibm.com>
To:     linux-ext4@...r.kernel.org
Cc:     jack@...e.com, tytso@....edu, Hillf Danton <hdanton@...a.com>,
        linux-fsdevel@...r.kernel.org,
        Ritesh Harjani <riteshh@...ux.ibm.com>,
        Borislav Petkov <bp@...en8.de>,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        syzbot+82f324bb69744c5f6969@...kaller.appspotmail.com
Subject: [PATCHv2 1/1] ext4: mballoc: Use this_cpu_read instead of this_cpu_ptr

Simplify reading a seq variable by directly using this_cpu_read API
instead of doing this_cpu_ptr and then dereferencing it.

This also avoid the below kernel BUG: which happens when
CONFIG_DEBUG_PREEMPT is enabled

BUG: using smp_processor_id() in preemptible [00000000] code: syz-fuzzer/6927
caller is ext4_mb_new_blocks+0xa4d/0x3b70 fs/ext4/mballoc.c:4711
CPU: 1 PID: 6927 Comm: syz-fuzzer Not tainted 5.7.0-next-20200602-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
Call Trace:
 __dump_stack lib/dump_stack.c:77 [inline]
 dump_stack+0x18f/0x20d lib/dump_stack.c:118
 check_preemption_disabled+0x20d/0x220 lib/smp_processor_id.c:48
 ext4_mb_new_blocks+0xa4d/0x3b70 fs/ext4/mballoc.c:4711
 ext4_ext_map_blocks+0x201b/0x33e0 fs/ext4/extents.c:4244
 ext4_map_blocks+0x4cb/0x1640 fs/ext4/inode.c:626
 ext4_getblk+0xad/0x520 fs/ext4/inode.c:833
 ext4_bread+0x7c/0x380 fs/ext4/inode.c:883
 ext4_append+0x153/0x360 fs/ext4/namei.c:67
 ext4_init_new_dir fs/ext4/namei.c:2757 [inline]
 ext4_mkdir+0x5e0/0xdf0 fs/ext4/namei.c:2802
 vfs_mkdir+0x419/0x690 fs/namei.c:3632
 do_mkdirat+0x21e/0x280 fs/namei.c:3655
 do_syscall_64+0x60/0xe0 arch/x86/entry/common.c:359
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

Fixes: 42f56b7a4a7d ("ext4: mballoc: introduce pcpu seqcnt for freeing PA
to improve ENOSPC handling")
Suggested-by: Borislav Petkov <bp@...en8.de>
Tested-by: Marek Szyprowski <m.szyprowski@...sung.com>
Signed-off-by: Ritesh Harjani <riteshh@...ux.ibm.com>
Reported-by: syzbot+82f324bb69744c5f6969@...kaller.appspotmail.com
---
 fs/ext4/mballoc.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/ext4/mballoc.c b/fs/ext4/mballoc.c
index a9083113a8c0..c0a331e2feb0 100644
--- a/fs/ext4/mballoc.c
+++ b/fs/ext4/mballoc.c
@@ -4708,7 +4708,7 @@ ext4_fsblk_t ext4_mb_new_blocks(handle_t *handle,
 	}
 
 	ac->ac_op = EXT4_MB_HISTORY_PREALLOC;
-	seq = *this_cpu_ptr(&discard_pa_seq);
+	seq = this_cpu_read(discard_pa_seq);
 	if (!ext4_mb_use_preallocated(ac)) {
 		ac->ac_op = EXT4_MB_HISTORY_ALLOC;
 		ext4_mb_normalize_request(ac, ar);
-- 
2.25.4