LOCK ORDERING(9)

🎀 lock ordering 🎀

okay bestie, welcome to the kernel API manual corner of the internet 💅 this page is literally just called lock ordering and it's about the two locks that are lowkey the main characters of the queuing routines: the qp s_lock and the exp_lock. that's it, that's the vibe check 🫶

🔒 the actual tea 🔒

so there are two locks involved with the queuing routines: the qp s_lock and the exp_lock. iconic duo, no cap ✨

here's the part where you gotta pay attention: since the tid space allocation gets called from the send engine, the qp s_lock is already held by the time you show up 😭 not new info, just how it is — like walking in and the aux is already on.

the allocation routines will get the exp_lock. that's their whole thing. they grab it 🫡

now, the first_qp call is provided so that the head of the rcd wait queue can be fetched under the exp_lock — and then, right after, the exp_lock gets dropped. fetch under, drop after. that's the whole choreography, periodt 💃

and one last thing, very important, don't forget it: any qp in the wait list will have the qp reference count held — held so the qp stays in memory. literally keeping it alive, main character energy 👑

ngl the lock ordering here is not a suggestion, it's the whole plot. respect the plot 🙈

✨ SEE ALSO ✨

go read these next, they're all connected, iykyk 💜

• the kernel file ./drivers/infiniband/hw/hfi1/tid_rdma.c
• TID RDMA READ protocol(9)
• qp_to_rcd(9)
• kernel_tid_waiters(9)
• dequeue_tid_waiter(9)
• queue_qp_for_tid_wait(9)
• __trigger_tid_waiter(9)
• tid_rdma_schedule_tid_wakeup(9)
• tid_rdma_trigger_resume(9)
• kern_reserve_flow(9)
• tid_rdma_find_phys_blocks_4k(9)
• tid_flush_pages(9)
• tid_rdma_find_phys_blocks_8k(9)
• hfi1_kern_exp_rcv_setup(9)
• hfi1_kern_exp_rcv_free_flows(9)
• __trdma_clean_swqe(9)
• hfi1_schedule_tid_send(9)

okay that's the page!! mwah 💖 this lil legend lives in ./drivers/infiniband/hw/hfi1 and it's dated October 2026 — yes, future girl vibes 🌸 go forth and hold your locks in the right order, bestie, i believe in you 🦋💅