[PATCH next v3 02/15] mtd: mtdoops: synchronize kmsg_dumper

[John Ogness <john.ogness@linutronix.de>]

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).

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

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);
-- 
2.20.1