x86/sgx: Break up long non-preemptible delays in sgx_vepc_release()

On large enclaves we hit the softlockup warning with following call trace:

	xa_erase()
	sgx_vepc_release()
	__fput()
	task_work_run()
	do_exit()

The latency issue is similar to the one fixed in:

  8795359e ("x86/sgx: Silence softlockup detection when releasing large enclaves")

The test system has 64GB of enclave memory, and all is assigned to a single VM.
Release of 'vepc' takes a longer time and causes long latencies, which triggers
the softlockup warning.

Add cond_resched() to give other tasks a chance to run and reduce
latencies, which also avoids the softlockup detector.

[ mingo: Rewrote the changelog. ]

Fixes: 540745dd ("x86/sgx: Introduce virtual EPC for use by KVM guests")
Reported-by: Yu Zhang <yu.zhang@ionos.com>
Signed-off-by: Jack Wang <jinpu.wang@ionos.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Tested-by: Yu Zhang <yu.zhang@ionos.com>
Reviewed-by: Jarkko Sakkinen <jarkko@kernel.org>
Reviewed-by: Kai Huang <kai.huang@intel.com>
Acked-by: Haitao Huang <haitao.huang@linux.intel.com>
Cc: stable@vger.kernel.org

arch/x86/kernel/cpu/sgx/virt.c

@@ -204,6 +204,7 @@ static int sgx_vepc_release(struct inode *inode, struct file *file)
 			continue;
 
 		xa_erase(&vepc->page_array, index);
+		cond_resched();
 	}
 
 	/*
@@ -222,6 +223,7 @@ static int sgx_vepc_release(struct inode *inode, struct file *file)
 			list_add_tail(&epc_page->list, &secs_pages);
 
 		xa_erase(&vepc->page_array, index);
+		cond_resched();
 	}
 
 	/*
@@ -243,6 +245,7 @@ static int sgx_vepc_release(struct inode *inode, struct file *file)
 
 		if (sgx_vepc_free_page(epc_page))
 			list_add_tail(&epc_page->list, &secs_pages);
+		cond_resched();
 	}
 
 	if (!list_empty(&secs_pages))