5.15-stable review patch. If anyone has any objections, please let me know.
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From: Jason Xing kernelxing@tencent.com
[ Upstream commit 4d4d9ef9dfee877d494e5418f68a1016ef08cad6 ]
Resolve the budget negative overflow which leads to returning true in ixgbe_xmit_zc even when the budget of descs are thoroughly consumed.
Before this patch, when the budget is decreased to zero and finishes sending the last allowed desc in ixgbe_xmit_zc, it will always turn back and enter into the while() statement to see if it should keep processing packets, but in the meantime it unexpectedly decreases the value again to 'unsigned int (0--)', namely, UINT_MAX. Finally, the ixgbe_xmit_zc returns true, showing 'we complete cleaning the budget'. That also means 'clean_complete = true' in ixgbe_poll.
The true theory behind this is if that budget number of descs are consumed, it implies that we might have more descs to be done. So we should return false in ixgbe_xmit_zc to tell napi poll to find another chance to start polling to handle the rest of descs. On the contrary, returning true here means job done and we know we finish all the possible descs this time and we don't intend to start a new napi poll.
It is apparently against our expectations. Please also see how ixgbe_clean_tx_irq() handles the problem: it uses do..while() statement to make sure the budget can be decreased to zero at most and the negative overflow never happens.
The patch adds 'likely' because we rarely would not hit the loop condition since the standard budget is 256.
Fixes: 8221c5eba8c1 ("ixgbe: add AF_XDP zero-copy Tx support") Signed-off-by: Jason Xing kernelxing@tencent.com Reviewed-by: Larysa Zaremba larysa.zaremba@intel.com Reviewed-by: Paul Menzel pmenzel@molgen.mpg.de Reviewed-by: Aleksandr Loktionov aleksandr.loktionov@intel.com Tested-by: Priya Singh priyax.singh@intel.com Signed-off-by: Tony Nguyen anthony.l.nguyen@intel.com Link: https://patch.msgid.link/20250819222000.3504873-4-anthony.l.nguyen@intel.com Signed-off-by: Jakub Kicinski kuba@kernel.org Signed-off-by: Sasha Levin sashal@kernel.org --- drivers/net/ethernet/intel/ixgbe/ixgbe_xsk.c | 4 +++- 1 file changed, 3 insertions(+), 1 deletion(-)
diff --git a/drivers/net/ethernet/intel/ixgbe/ixgbe_xsk.c b/drivers/net/ethernet/intel/ixgbe/ixgbe_xsk.c index b399b9c14717..9a789a419166 100644 --- a/drivers/net/ethernet/intel/ixgbe/ixgbe_xsk.c +++ b/drivers/net/ethernet/intel/ixgbe/ixgbe_xsk.c @@ -392,7 +392,7 @@ static bool ixgbe_xmit_zc(struct ixgbe_ring *xdp_ring, unsigned int budget) dma_addr_t dma; u32 cmd_type;
- while (budget-- > 0) { + while (likely(budget)) { if (unlikely(!ixgbe_desc_unused(xdp_ring))) { work_done = false; break; @@ -427,6 +427,8 @@ static bool ixgbe_xmit_zc(struct ixgbe_ring *xdp_ring, unsigned int budget) xdp_ring->next_to_use++; if (xdp_ring->next_to_use == xdp_ring->count) xdp_ring->next_to_use = 0; + + budget--; }
if (tx_desc) {