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From:   Michael Ellerman <mpe@ellerman.id.au>
To:     linuxppc-dev@ozlabs.org
Cc:     hch@lst.de, jk@ozlabs.org, viro@zeniv.linux.org.uk,
        linux-kernel@vger.kernel.org
Subject: [PATCH] powerpc/spufs: Add rcu_read_lock() around fcheck()
Date:   Tue, 28 Apr 2020 21:48:11 +1000
Message-Id: <20200428114811.68436-1-mpe@ellerman.id.au>
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Sender: linux-kernel-owner@vger.kernel.org
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Currently the spu coredump code triggers an RCU warning:

  =============================
  WARNING: suspicious RCU usage
  5.7.0-rc3-01755-g7cd49f0b7ec7 #1 Not tainted
  -----------------------------
  include/linux/fdtable.h:95 suspicious rcu_dereference_check() usage!

  other info that might help us debug this:

  rcu_scheduler_active = 2, debug_locks = 1
  1 lock held by spu-coredump/1343:
   #0: c0000007fa22f430 (sb_writers#2){.+.+}-{0:0}, at: .do_coredump+0x1010/0x13c8

  stack backtrace:
  CPU: 0 PID: 1343 Comm: spu-coredump Not tainted 5.7.0-rc3-01755-g7cd49f0b7ec7 #1
  Call Trace:
    .dump_stack+0xec/0x15c (unreliable)
    .lockdep_rcu_suspicious+0x120/0x144
    .coredump_next_context+0x148/0x158
    .spufs_coredump_extra_notes_size+0x54/0x190
    .elf_coredump_extra_notes_size+0x34/0x50
    .elf_core_dump+0xe48/0x19d0
    .do_coredump+0xe50/0x13c8
    .get_signal+0x864/0xd88
    .do_notify_resume+0x158/0x3c8
    .interrupt_exit_user_prepare+0x19c/0x208
    interrupt_return+0x14/0x1c0

This comes from fcheck_files() via fcheck().

It's pretty clearly documented that fcheck() must be wrapped with
rcu_read_lock(), so fix it.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
---
 arch/powerpc/platforms/cell/spufs/coredump.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/arch/powerpc/platforms/cell/spufs/coredump.c b/arch/powerpc/platforms/cell/spufs/coredump.c
index 8b3296b62f65..0fc52cbaa552 100644
--- a/arch/powerpc/platforms/cell/spufs/coredump.c
+++ b/arch/powerpc/platforms/cell/spufs/coredump.c
@@ -82,13 +82,19 @@ static int match_context(const void *v, struct file *file, unsigned fd)
  */
 static struct spu_context *coredump_next_context(int *fd)
 {
+	struct spu_context *ctx;
 	struct file *file;
 	int n = iterate_fd(current->files, *fd, match_context, NULL);
 	if (!n)
 		return NULL;
 	*fd = n - 1;
+
+	rcu_read_lock();
 	file = fcheck(*fd);
-	return SPUFS_I(file_inode(file))->i_ctx;
+	ctx = SPUFS_I(file_inode(file))->i_ctx;
+	rcu_read_unlock();
+
+	return ctx;
 }
 
 int spufs_coredump_extra_notes_size(void)
-- 
2.25.1