linux-kernel - [PATCH 4.19 27/37] iwlwifi: pcie: reschedule in long-running memory reads

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Date:   Tue,  2 Feb 2021 14:39:10 +0100
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org, Johannes Berg <johannes.berg@...el.com>,
        Luca Coelho <luciano.coelho@...el.com>,
        Kalle Valo <kvalo@...eaurora.org>,
        Sasha Levin <sashal@...nel.org>
Subject: [PATCH 4.19 27/37] iwlwifi: pcie: reschedule in long-running memory reads

From: Johannes Berg <johannes.berg@...el.com>

[ Upstream commit 3d372c4edfd4dffb7dea71c6b096fb414782b776 ]

If we spin for a long time in memory reads that (for some reason in
hardware) take a long time, then we'll eventually get messages such
as

  watchdog: BUG: soft lockup - CPU#2 stuck for 24s! [kworker/2:2:272]

This is because the reading really does take a very long time, and
we don't schedule, so we're hogging the CPU with this task, at least
if CONFIG_PREEMPT is not set, e.g. with CONFIG_PREEMPT_VOLUNTARY=y.

Previously I misinterpreted the situation and thought that this was
only going to happen if we had interrupts disabled, and then fixed
this (which is good anyway, however), but that didn't always help;
looking at it again now I realized that the spin unlock will only
reschedule if CONFIG_PREEMPT is used.

In order to avoid this issue, change the code to cond_resched() if
we've been spinning for too long here.

Signed-off-by: Johannes Berg <johannes.berg@...el.com>
Fixes: 04516706bb99 ("iwlwifi: pcie: limit memory read spin time")
Signed-off-by: Luca Coelho <luciano.coelho@...el.com>
Signed-off-by: Kalle Valo <kvalo@...eaurora.org>
Link: https://lore.kernel.org/r/iwlwifi.20210115130253.217a9d6a6a12.If964cb582ab0aaa94e81c4ff3b279eaafda0fd3f@changeid
Signed-off-by: Sasha Levin <sashal@...nel.org>
---
 drivers/net/wireless/intel/iwlwifi/pcie/trans.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/drivers/net/wireless/intel/iwlwifi/pcie/trans.c b/drivers/net/wireless/intel/iwlwifi/pcie/trans.c
index fe772b716a8df..fcda33482887b 100644
--- a/drivers/net/wireless/intel/iwlwifi/pcie/trans.c
+++ b/drivers/net/wireless/intel/iwlwifi/pcie/trans.c
@@ -2127,6 +2127,7 @@ static int iwl_trans_pcie_read_mem(struct iwl_trans *trans, u32 addr,
 	while (offs < dwords) {
 		/* limit the time we spin here under lock to 1/2s */
 		unsigned long end = jiffies + HZ / 2;
+		bool resched = false;
 
 		if (iwl_trans_grab_nic_access(trans, &flags)) {
 			iwl_write32(trans, HBUS_TARG_MEM_RADDR,
@@ -2137,10 +2138,15 @@ static int iwl_trans_pcie_read_mem(struct iwl_trans *trans, u32 addr,
 							HBUS_TARG_MEM_RDAT);
 				offs++;
 
-				if (time_after(jiffies, end))
+				if (time_after(jiffies, end)) {
+					resched = true;
 					break;
+				}
 			}
 			iwl_trans_release_nic_access(trans, &flags);
+
+			if (resched)
+				cond_resched();
 		} else {
 			return -EBUSY;
 		}
-- 
2.27.0