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ice: set in_use only after preparing Tx timestamp index

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The ice_ptp_request_ts() function is used to request a timestamp index for
use with a packet. When reserving an index, it sets the start time and
saves a pointer to the skb into the appropriate index. The function marks
the in_use bit first before doing any of these steps. The IRQ handler which
clears the timestamps reads the in_use bits uses a lockless flow for
reading the in_use bits to determine which ones are in-use. This is
necessary as actually processing a complete timestamp must be able to sleep
so we cannot hold the timestamp tracker lock over the entire sequence.
Additionally, blocking the Tx hotpath with such a lock indefinitely would
be problematic.

However, the existing flow now has a very narrow window where the IRQ
handler could see a timestamp as in-use but read a stale value for its
"start" time.

Fix this by ordering the sequence to mark the in_use bit last, and add a
memory barrier to prevent re-ordering of the previous writes to setup the
index.

This was found and reported by Sashiko while reviewing an unrelated change.

Closes: https://sashiko.dev/#/patchset/20260821-jk-e825c-minimized-fixes-v1-0-9d0731eb4858%40intel.com?part=7
Fixes: ea9b847cda ("ice: enable transmit timestamps for E810 devices")
Signed-off-by: Jacob Keller <jacob.e.keller@intel.com>
Link: https://patch.msgid.link/20261027-reduction-v1-2-81237c67a361@intel.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
This commit is contained in:
Jacob Keller 2026-10-27 17:05:38 -07:00 committed by Jakub Kicinski
parent d5d859a6ef
commit 50870d6211
1 changed files with 10 additions and 2 deletions
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12
drivers/net/ethernet/intel/ice/ice_ptp.c
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@ -595,6 +595,12 @@ static void ice_ptp_process_tx_tstamp(struct ice_ptp_tx *tx)
bool drop_ts = !link_up;
struct sk_buff *skb;
/* Paired with smp_mb__before_atomic() in ice_ptp_request_ts()
* to ensure ordering the in_use bitmap and the indexed fields
* in the tstamps array.
*/
smp_rmb();
/* Drop packets which have waited for more than 2 seconds */
if (time_is_before_jiffies(tx->tstamps[idx].start + 2 * HZ)) {
drop_ts = true;
@ -2678,11 +2684,13 @@ s8 ice_ptp_request_ts(struct ice_ptp_tx *tx, struct sk_buff *skb)
* a reference to the skb and the start time to allow discarding old
* requests.
*/
set_bit(idx, tx->in_use);
clear_bit(idx, tx->stale);
tx->tstamps[idx].start = jiffies;
tx->tstamps[idx].skb = skb_get(skb);
skb_shinfo(skb)->tx_flags |= SKBTX_IN_PROGRESS;
clear_bit(idx, tx->stale);
/* Ensure index is setup before marking it as used */
smp_mb__before_atomic();
set_bit(idx, tx->in_use);
ice_trace(tx_tstamp_request, skb, idx);
}