[next,v4,02/15] mtd: mtdoops: synchronize kmsg_dumper
diff mbox series

Message ID 20210303101528.29901-3-john.ogness@linutronix.de
State Accepted
Commit 40ddbbac7f16eb91f9f2bbc414f283102052ce14
Headers show
Series
  • printk: remove logbuf_lock
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Commit Message

John Ogness March 3, 2021, 10:15 a.m. UTC
The kmsg_dumper can be called from any context and CPU, possibly
from multiple CPUs simultaneously. Since the writing of the buffer
can occur from a later scheduled work queue, the oops buffer must
be protected against simultaneous dumping.

Use an atomic bit to mark when the buffer is protected. Release the
protection in between setting the buffer and the actual writing in
order for a possible panic (immediate write) to be written during
the scheduling of a previous oops (delayed write).

An atomic bit (rather than a spinlock) was chosen so that no
scheduling or preemption side-effects would be introduced. The MTD
kmsg_dumper may dump directly or it may be delayed (via scheduled
work). Depending on the context, different MTD callbacks are used.
For example, mtd_write() expects to be called in a non-atomic
context and may take a mutex.

Signed-off-by: John Ogness <john.ogness@linutronix.de>
Reviewed-by: Petr Mladek <pmladek@suse.com>
---
 drivers/mtd/mtdoops.c | 12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

Comments

Miquel Raynal March 11, 2021, 9:50 a.m. UTC | #1
Hello,

John Ogness <john.ogness@linutronix.de> wrote on Wed,  3 Mar 2021
11:15:15 +0100:

> The kmsg_dumper can be called from any context and CPU, possibly
> from multiple CPUs simultaneously. Since the writing of the buffer
> can occur from a later scheduled work queue, the oops buffer must
> be protected against simultaneous dumping.
> 
> Use an atomic bit to mark when the buffer is protected. Release the
> protection in between setting the buffer and the actual writing in
> order for a possible panic (immediate write) to be written during
> the scheduling of a previous oops (delayed write).
> 
> An atomic bit (rather than a spinlock) was chosen so that no
> scheduling or preemption side-effects would be introduced. The MTD
> kmsg_dumper may dump directly or it may be delayed (via scheduled
> work). Depending on the context, different MTD callbacks are used.
> For example, mtd_write() expects to be called in a non-atomic
> context and may take a mutex.
> 
> Signed-off-by: John Ogness <john.ogness@linutronix.de>
> Reviewed-by: Petr Mladek <pmladek@suse.com>

Reviewed-by: Miquel Raynal <miquel.raynal@bootlin.com>

Thanks,
Miquèl

Patch
diff mbox series

diff --git a/drivers/mtd/mtdoops.c b/drivers/mtd/mtdoops.c
index 774970bfcf85..8bbfba40a554 100644
--- a/drivers/mtd/mtdoops.c
+++ b/drivers/mtd/mtdoops.c
@@ -52,6 +52,7 @@  static struct mtdoops_context {
 	int nextcount;
 	unsigned long *oops_page_used;
 
+	unsigned long oops_buf_busy;
 	void *oops_buf;
 } oops_cxt;
 
@@ -180,6 +181,9 @@  static void mtdoops_write(struct mtdoops_context *cxt, int panic)
 	u32 *hdr;
 	int ret;
 
+	if (test_and_set_bit(0, &cxt->oops_buf_busy))
+		return;
+
 	/* Add mtdoops header to the buffer */
 	hdr = cxt->oops_buf;
 	hdr[0] = cxt->nextcount;
@@ -190,7 +194,7 @@  static void mtdoops_write(struct mtdoops_context *cxt, int panic)
 				      record_size, &retlen, cxt->oops_buf);
 		if (ret == -EOPNOTSUPP) {
 			printk(KERN_ERR "mtdoops: Cannot write from panic without panic_write\n");
-			return;
+			goto out;
 		}
 	} else
 		ret = mtd_write(mtd, cxt->nextpage * record_size,
@@ -203,6 +207,8 @@  static void mtdoops_write(struct mtdoops_context *cxt, int panic)
 	memset(cxt->oops_buf, 0xff, record_size);
 
 	mtdoops_inc_counter(cxt);
+out:
+	clear_bit(0, &cxt->oops_buf_busy);
 }
 
 static void mtdoops_workfunc_write(struct work_struct *work)
@@ -276,8 +282,11 @@  static void mtdoops_do_dump(struct kmsg_dumper *dumper,
 	if (reason == KMSG_DUMP_OOPS && !dump_oops)
 		return;
 
+	if (test_and_set_bit(0, &cxt->oops_buf_busy))
+		return;
 	kmsg_dump_get_buffer(dumper, true, cxt->oops_buf + MTDOOPS_HEADER_SIZE,
 			     record_size - MTDOOPS_HEADER_SIZE, NULL);
+	clear_bit(0, &cxt->oops_buf_busy);
 
 	if (reason != KMSG_DUMP_OOPS) {
 		/* Panics must be written immediately */
@@ -394,6 +403,7 @@  static int __init mtdoops_init(void)
 		return -ENOMEM;
 	}
 	memset(cxt->oops_buf, 0xff, record_size);
+	cxt->oops_buf_busy = 0;
 
 	INIT_WORK(&cxt->work_erase, mtdoops_workfunc_erase);
 	INIT_WORK(&cxt->work_write, mtdoops_workfunc_write);